Friction elements especially resistant to wear by abrasion



United States Patent 3,321,338 FRICTION ELEMENTS ESPECIALLY RESISTANT TOWEAR BY ABRASION Jacques Caubet, Saint-Etienne, France, assignor toAutomobiles M. Berliet, Lyon, France, and Hydromecanique et Frottement,Saint-Etienne, France, both French corporations No Drawing. Filed Dec.10, 1964, Ser. No. 417,495 Claims priority, application France, Dec. 11,1963, 956,874, Patent 1,386,446; Feb. 27, 1964, 965,390, Patent 85,297;Feb. 28, 1964, 965,616, Patent 85,370; June 22, 1964, 979,152, Patent85,994; June 23, 1964, 979,316, Patent 86,012

7 Claims. (Cl. 14811.5)

For a long time past, efforts have been made to produce frictionelements, the rate of wear of which would remain low with no or verydoubtful lubrication, and in the presence of abrasive grains. Thispreoccupation has been common to numerous industries which have foundthemselves facing the problems presented, for example, by the wear ofparts such as gears and bearings working in the atmosphere of cementworks, the oscillation shafts of springs on railway rolling stock, theoscillation shafts of the suspension of road vehicles or of public worksmachines, mining equipment, amongst others.

The properties of resistance to Wear of hypereutectoid steels containingat least 12% of manganese, commonly known as Hadfield steels have beenknown for a very long time. At the present time, these steels aregenerally employed in the condition of super-temper, that is to say witha stable austenitic structure and then, under the effect of the forcesdue to friction, to abrasion or to shocks, the superficial austenitebecomes converted to a kind of very hard martensite which has aremarkable resistance to wear.

However, this method of utilization is subject to two conditions whichlimit its use. On the one hand, the initial forces must exceed a certainthreshold below which the cold-hammering does not take place, and on theother hand, an initial wear must be permitted which is generally veryconsiderable during the period of time necessary for the formation ofthe cold-hammered layer. An example of the first disadvantage is givenby the balls of ball-mills, which have a good resistance in cementball-mills, but remain austenitic and Wear rapidly in plaster mills.

The second disadvantage makes it impossible to utilize Hadfield steelsfor parts, the precise initial dimensions of which must be preserved asfar as possible.

Similar phenomena are observed on austenitic stainless steels, such asfor example the common stainless steels comprising substantialproportions of addition elements such as chromium and nickel, and on thehypereutectoid alloy steels containing by weight 11 to 14% of magnanese,l to 1.3% of carbon, not more than 0.6% of silicon, impurities in theusual proportions, the remainder being constituted by iron.

Finally, as regards hardening, titanium and its alloys exhibit similarproperties together with the drawbacks which result from them.

All the metals and alloys enumerated above have the common property ofhardening by cold-hammering, subsequently showing a great density ofplanes of slip in their superficial crystalline structure.

Starting from a metallic material having the above properties, theobject of the present invention is to obtain parts which presentimmediately, as soon as they are put to use, a good resistance toseizure and to wear.

According to the invention, parts made of a metallic material capable ofhardening by cold-hammering or coldrolling and having in its superficialcrystalline structure a high density of plane of slip, to a deepcold-hammer- 3,321,338 Patented May 23, 1967 ing carried out in suchmanner that it produces on the surface of the parts striations which aresubstantially perpendicular to the direction of application of thefriction. After the cold-hammering or cold-rolling operation, there iseffected a superficial incorporation of at least one element chosen fromthe groups of metalloids and transition metals, metalloids andtransition metals possessing one of two properties, one of whichconsists of the capacity to deposit on the surface of the parts an ioniccompound produced by reaction with the underlying metal, while the otherhas the capacity of inserting its atoms in the structure of theunderlying metal, forming a solid insertion solution. This incorporationis obtained by means of at least one thermal treatment effected in afluid, such as a salt bath or a gaseous atmosphere, which contains theelement to be incorporated.

The cold-hammering can be carried out either by knurling or byhammering. In the case of knurling, which is preferably used when theparts to be cold-hammered are flat or round surfaces, the rollers mustbe cut so that the striations which they impress by forging on thesurface of the part are arranged in a direction substantiallyperpendicular to the direction of application of the friction. In thecase where the parts to be cold-hardened are neither flat nor round, itis preferable to employ a hammer with a striated striking face.

There will be employed with advantage as an approximate minimumcold-hardening load on a roller a diameter 20 mm. and a length of 10mm., a load substantially equal in kilopond (9.81 Newton) to the elasticlimit of the material to be cold-hardened, in kp./ sq. mm. For theapproximate maximum charge, there will preferably be chosen that which,taking account of the geometric form of the part to be cold-hardened andassuming the roller to be smooth, gives hertzian pressures equal tothree times the elastic limit of the material.

The thermal treatment or treatments are carried out in such manner thatthere is obtained a superficial incorporation of one or of twometalloids, or of a transition metal capable either of depositing on thesurface of the part an ionic compound by reaction with the underlyingmetal, or of inserting its atoms into the" structure of the underlyingmetal while forming a solid insertion solution. These transition metalsor these metalloids may be, for example, nitrogen, sulphur, selenium,'telluriu-m, amongst others.

The introduction of a metalloid or of a transition metal as definedabove, into the cold-hardened layer is obtained by a heat treatment ofthe knurled part in a salt bath or in gaseous atmosphere for a period oftime anywhere from one hour to as long as six hours at a temperaturebetween 400 C. and 590 C.

For the superficial incorporation of sulphur, the coldhardened part istreated with a bath of sulphur salts or a gaseous sulphur atmosphere fora period of time of at least two hours and at a temperature higher than500 C.

A treatment of this kind can for example be ap lied by means of aprocess known in France under the commercial name of Sulfinuz. However,all the identical or similar processes using salt baths or a gaseousmedium producing superficial structure layers of compositions similar oridentical with those which are obtained by the Sulfinuz process may beemployed. 3

In the case of the Sulfinuz process, the parts are preheated to about300 to 350 C. and are then immersed for a period of 2 to 6 hours in asalt bath maintained at an approximate temperature of 570 C. and arecomposed of an inactive base such as the alkali and alkaline earthchlorides and carbonates, permitting a melting point to be obtained ofless than 500 C. of sulphur compounds, the action of which ispreponderant and buffer cyanides or cyanates which protect the sulphurcompounds by keeping the bath in a reducing medium. In certainparticular cases, the treatment temperature in salt baths or in agaseous atmosphere may, depending on the composition of the bath, forexample when it contains a sulphide and a ferro-cy-anide, be less than570 C., the sulphuretting action being then capable of taking place from400 C.

The approximate composition of a Sulfinuz bath is given below by way .ofexample:

7 Percent Sulphide 0.5 Alkaline cyanides 8 Alkaline cyanates 27 Alkalinechlorides 33 Alkaline carbonates 31.5

As regards ferrous alloys, the incorporation of nitrogen can be effectedby means of any nitriding process which is effective below a maximumtemperature of 580 C., beyond which temperature the decomposition of theaustenite is too great and introduces excessive brittleness.

According to the invention, it is also possible to make use of theprocess known by the commercial name of Tenifer or of all identical orsimilar processes using salt baths or gaseous media producingsuperfiicial layers of structures and of compositions similar oridentical to those which are obtained by the Tenifer process.

The latter, also known in the anglo-saxon countries by the name ofTufftride is a process of mild nitridation which makes it possible toobtain on the steel parts, an outer layer comprising iron carbide andiron nitride covering a ditfusionlayer of nitrogen in the steel. Thisresult can be obtained for example by immersing the part for asufficient pre-determined period, for example for two hours, in a saltbath heated to a temperature comprised between 550 and 580 C., forerample 565 C., comprising about 32 to 35% of alkali cyanates, forexample 45 of potassium cyanate, and 50 to 55% approx. of alkalicyanides, for example 55 of potassium cyanide, the bath being stirred byblowing-in air.

The results obtained are substantially identical with those which arerecorded after a sulphurizing treatment, such as the Sulfinuz process.

Furthermore, and again in accordance with the invention, thecold-hardened parts can advantageously be successively subjected to anitridation such as the Tenifer treatment and then to a sulphurizingtreatment, for example according to the Sulfinuz process.

Various possibilities of the application of the method according to theinvention will be brought out from the following examples:

Example 1 A shaft and rings of 15 40 mm. were machined from a steel inthe super-tempered state containing, by weight, 1.2% of carbon and 14.5%of manganese. Immediately after the lathe operation, the bearingsurfaces of the shaft and the corresponding surfaces of the rings wereknurled with a tool which pressed two rollers of 20 mm. against theparts with a force of 400 kg., giving two families of striations of 0.15mm. in depth and 1 mm. apart, respectively inclined by -15 to thedirection of the gen erator lines.

The knurled wheels made 14 to-and-fro passages under these conditions.

As the knurling operation causes a slight swelling of about 0.05 mm.,the part is easily brought back to its initial dimension by a lightpolishing with emery cloth.

The parts, shaft and rings, cold-hardened in this way, have in sectionunder metallographic examination, extremely dense networks of lines ofshear, this is to say having for example four lines of shear per squareof 0.01 mm. sides, and penetrating to a depth of more than 0.3 mm. Themicro-hardness indicates that in the zone of shear, the initial hardnessof 70 kg./sq. mm. has increased to 190 kg./sq. mm., the hardness of thecore not being modified.

The parts were then immersed for 3 hours in a sulphurizing and cyanidingbath of the Sulfinuz type at a temperature of 585 i-S C. After thistreatment, a first examination of metallography and micro-hardness showsthat the core-hardness has passed from 70 to 125 kg./ sq. mm., thisincrease being due to the conventional decomposition of the austeniteand the precipitation of the carbides of iron and manganese. In thesuperficial zone of the lines of shear obtained by knurling, thehardness remains unchanged, and all the old shear lines, even thedeepest, are the centres of fine and dense precipitations.

The tests of the shaft and the rings thus treated show performanceswhich are extraordinarily improved and without common measure with thosewhich are obtained, all other things being equal, either withnon-Sulfinuzed knurled Hadfield steel or with non-knurled SulfinuzedHadfield steel.

In the example described above, the shaft of '40 mm. oscillated in aring of 40 mm. in length with a clearance of 0.1 mm. on the diameter.The whole was assembled without lubrication and immersed in water. Theamplitude of the oscillations was 90 and their frequency was 2cycles/sec. The load was 2,000 kp.

Under these conditions, a shaft and a ring of knurled non-SulfinuzedHadfield steel showed traces of seizure with a great increase in thecoefficient of friction at the end of 12 hours. A ring and a shaft ofnon-knurled but Sulfinuzed Hadfield steel showed defects by scaling atthe end of 30 hours; on the other hand, the shaft and the ring treatedaccording to the example of the process according to the invention wereable to oscillate for 500 hours, while retaining a perfect state ofsurface and without variation of the coefiicient of friction.

Example 2 A part of Hadfield steel, cold-hardened and knurled, wasplunged into a bath of molten salts at 560 C., containing cyanides andunstable isomers of potassium cyanate. After immersion in this bath, ofwhich one of the types is known by the commercial name of Tenifer, thesame precipitates are found along the planes of shear, without increasein the micro-hardness, over the first four-tenths of a millimeter of thesurface of the part, as those which were observed on the cold-hardenedparts subjected to a sulphurizing treatment.

Example 3 A shaft and rings of the same dimensions as those of Example 1were machined from a standard stainless steel containing, by weight, 18%of chromium, 8% of nickel and 3% of molybdenum. Tested under thetreatments according to the invention, substantially under the sameconditions of tests as the parts of Example 1, that is to say with anamplitude of oscillations of a frequency of one cycle per sec. and aload of 2,000 kp., the whole immersed in water at 20 C., seizureinevitably occurs in less than a few minutes. On the other hand, whenknurled and subjected to the sulphurizing treatment, an identical shaftcan oscillate for more than 50 hours while retaining a satisfactorysurface condition. The same advantage was obtained by means ofnitridation.

Example 4 Parts of titanium or of titanium alloy were subjected tocold-hardening followed in some cases by a sulphurizing thermaltreatment and in the other cases by a nitriding treatment. After theseapplications of the process according to the invention, they all showedequally as soon as they were put into use, a good resistance to seizureand to wear.

Thus, in a titanium alloy of the type Ti A, in accordance with Frenchstandards, there were taken two male and female oscillation test samplessimilar to those described in Example 1. These samples were knurledaccording to the invention with wheels of 20 mm. and 10 mm. in width,the striations being inclined by at least 60 to the direction of slip.The wheels must necessarily be made of high-speed steel previouslySulfinuzed so as to prevent them from adhering to the titanium. WhenSulfinuzed, samples of this kind have been capable of oscillatingwithout incident for 10 hours under a load of 200 kp., the amplitudebeing 90, the frequency one cycle/sec, the temperature ambient and themedium, water. Under the same conditions, the same sample pieces oftitanium not treated according to the invention, seize-up instantly.

Example 5 A piece of Hadfield steel, after the cold-hardening operation,is immersed for two hours in a bath of nitriding salts at 560 C., thebath being constituted by a mixture of alkali cyanates and cyanidesfollowing the Tenifer process. After this mild nitridation, the part issubjected to the sulphurizing action of a bath such as that of theprocess known "by the commercial name of Sulfinuz, this processcomprising a treatment of at least two hours in a sulphurizing 'bath ora sulphurizing atmosphere at a temperature exceeding 500 C. The resultsobtained are superior to those which are recorded on cold-hardened partssubjected only either to a sulphurizing treatment or to a nitridingtreatment.

What is claimed is:

1. A method for producing metallic parts having a high resistance toseizure and to wear by friction, wherein parts made from a metallicmaterial chosen from the group of metals capable of hardening bycold-hammering While exhibiting in its superficial crystalline structurea high density of planes of shear, and consisting of austeniticstainless steels, hyper-eutectoid alloy steels containing at least 11%by weight of manganese, titanium, and titanium alloys containing a majorpart of titanium, are subjected to a deep cold-hardening executed insuch manner that it produces, on the surface of the part, striationssubstantially perpendicular to the direction of the friction, followingwhich the part is subjected to the superficial incorporation of at leastone element chosen from the group of the metalloids and transitionmetals, which metalloids and transition metals possess one of the twoproperties consisting firstly in the capacity of depositing at thesurface of the parts an ionic compound by reaction with the underlyingmetal, and secondly of the capacity of inserting its atoms into thestructure of the underlying metal while forming a solid insertionsolution, the metalloid group consisting of nitrogen, carbon and sulphurand the transition metal group of selenium and tellurium, thesuperficial incorporation being obtained by maintaining a contact duringa period comprised between 1 and 6 hours at a temperature from 400 to590 C. between said part and a fluid such as a salt bath or a gaseousatmosphere, which contains the element to be incorporated.

2. A method for producing metallic parts having a high resistance toseizure and to wear by friction which comprises forming a metallic partof a metal capable of hardening by cold-hammering while exhibiting inits superficial crystalline structure a high density of planes of shearand selected from the group consisting of austenitic stainless steels,hyper-eutectoid alloy steels containing at least 11% by weightmanganese, titanium, and titanium alloys containing a major part oftitanium; subjecting the part to a deep cold-hardening executed in sucha manner that it produces on the surface of the part striationssubstantially perpendicular to the direction of frictional wear;subjecting the striated part to superficial incorporation of at leastone element selected from the group consisting of nitrogen, carbon,sulfur, selenium, and tellurium, by maintaining contact between saidmetal part and a fluid containing such element for a period of timebetween one and six hours at a temperature of about 400 to 590 C.

3. A method in accordance with claim 1 wherein the deep cold-hardeningis eifected by knurling.

4. A method according to claim 1 wherein the deep cold-hardening iseffected 'by hammering with a striated hammer.

5. A method in accordance with claim 1, wherein the case where theelement is sulphur, the parts are subjected to a thermal treatment of atleast two hours in sulphurizing bath or a sulphurizing atmosphere at atemperature higher than 500 C.

6. A method in accordance with claim 1 wherein in the case ofincorporation of nitrogen, the said incorporation is obtained by meansof any nitriding process which is eflt'ective below a maximumtemperature of 580 C.

7. A method in accordance with claim 6, wherein after the nitridation,the part is subjected to a treatment for at least two hours in asulphurizing bath at a temperature of at least 400 C.

References Cited by the Examiner UNITED STATES PATENTS 2,707,159 4/1955Foucry et al. 1486.24 X 3,009,843 1l/l961 Nachtman et al. l48l2.13,022,204 2/ 1962 Muller et al. l48l5.5 3,208,885 9/1965 Muller 14815.5

FOREIGN PATENTS 782,263 9/1957 Great Britain.

DAVID L. RE'CK, Primary Examiner.

H. F. SAITO, Assistant Examiner.

1. A METHOD FOR PRODUCING METALLIC PARTS HAVING A HIGH RESISTANCE TOSEIZURE AND TO WEAR BY FRICTION, WHEREIN PARTS MADE FROM A METALLICMATERIAL CHOSEN FROM THE GROUP OF METALS CAPABLE OF HARDENINGBYCOLD-HAMMERING WHILE EXHIBITING IN ITS SUPERFICIAL CRYSTALLINESTRUCTURE A HIGH DENSITYOF PLANES OF SHEAR, AND CONSISTING OF AUSTENITICSTAINLESS STEELS, HYPER-EUTECTOID ALLOY STEELS CONTAINING AT LEAST 11%BY WEIGHT OF MANGANESE, TITANIUM, AND TITANIUM ALLOYS CONTAINING AMOJORPART OF TITANIUM, ARE SUBJECTED TO A DEEP COLD-HARDENING EXECUTED INSUCH MANNER THAT IT PRODUCES, ON THE SURFACE OF THE PART, STRIATIONSSUBSTANTIALLY PERPENDICULAR TO THE DIRECTION OF THE FRICTION, FOLLOWINGWHICH THE PART IS SUBJECTED TO THE SUPERFICIAL INCORPORATION OF AT LEASTONE ELEMENT CHOSEN FROM THE GROUP OF THE METALLOIDS AND TRANSITIONMETALS, WHICH METALLOIDS AND TRANSITION METALS POSSESS ONE OF THE TWOPROPERTIES CONSISTING FIRSTLY IN THE CAPACITY OF DEPOSITING AT THESURFACE OF THE PARTS AN IONIC COMPOUND BY REACTION WITH THE UNDERLYINGMETAL, AND SECONDLY OF THE CAPACITY OF INSERTING ITS ATOMS INTO THESTRUCTURE OF THE UNDERLYING METAL WHILE FORMING A SOLID INSERTIONSOLUTION, THE METALLOID GROUP CONSISTING OF NITROGEN, CARON AND SULPHURAND THE TRANSITION METAL GROUP OF SELENIUM AND TELLURIUM, THESUPERFICIAL INCORPORATION BEING OBTAINED BY MAINTAINING A CONTACT DURINGA PERIOD COMPRISED BETWEEN 1 AND 6 HOURS AT A TEMPERATURE FROM 400 TO590*C. BETWEEN SAID PART AND A FLUID SUCH AS A SALT BATH OR A GASEOUSATMOSPHERE, WHICH CONTAINS THE ELEMENT TO BE INCORPORATED.